Prepare 0.6.0-beta.1 release #159

Job	Run time
fmt	31s
clippy	1m 15s
build (stable, --features "")	1m 17s
test	2m 7s
build (stable, --all-features)	4m 10s
build (stable, --no-default-features)	2m 7s
build (1.80, --features "")	1m 49s
build (1.80, --all-features)	3m 13s
build (1.80, --no-default-features)	1m 27s
build (beta, --features "")	1m 43s
build (beta, --all-features)	3m 24s
build (beta, --no-default-features)	1m 26s
build (nightly, --features "")	1m 49s
build (nightly, --all-features)	1m 38s
build (nightly, --no-default-features)	1m 29s
	29m 25s

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